Superconducting magnets conduct electricity with effectively zero resistance as long as the magnets are maintained at a suitably low temperature, which is referred to as a “superconducting temperature” hereinafter. Cryogenic systems are used to ensure that the superconducting magnets work at the superconducting temperature.
Superconducting magnets generally comprise superconducting coils electrically coupled to a power supply through current leads for transmitting electrical current to the superconducting coils. These current leads each include one end electrically coupled to the superconducting coil, and another end electrically coupled to the power supply. The superconducting magnet is coupled to the power supply during a ramp operation to power or charge the magnet to a specified field, then put it into a persistent mode. Cryogenic devices, such as superconducting magnets, may require current ranging from a hundred to several thousand amperes to be brought into the cold region of the cryostat. Significant heat is generated from the current leads that may inevitably propagate to the superconducting coils. Therefore, current leads must be designed to minimize the heat flow or loss into the cold region.
Some current lead assemblies include demountable current leads where the connection to the power supply is terminated when the magnet is powered and placed in persistent mode by disengaging or demounting the contacts of the current leads. The use of demountable current leads in a conduction cooled superconducting magnet system is complicated by the requirement for cooling the current lead that is coupled to the superconducting coils. In a conduction cooled superconducting magnet system, connection of the leads occurs in a vacuum chamber at low temperature, such as 50° K, for example. Electrical contacts made at these low temperatures in a vacuum, typically result in high contact resistance, possibly from frozen contaminants that are deposited to the contacts, as a result of outgassing of the materials in the vacuum. Connection at low temperatures also results in high resistance and high heat load because good contact is difficult to achieve since the contacts are very cold and rigid and have little compliance which makes it difficult to establish electrical contact. Accordingly, the leads serve as a heat load on the superconducting magnet. Therefore, a cooling apparatus typically is used for cooling the end of the current leads that is electrically coupled to the superconducting coils, which is often referred to as a “cold end”. The other end that is electrically coupled to the power supply is often referred to as a “warm end”. Establishing good electrical connection and minimizing heat load in conduction cooled superconducting magnets continues to be a challenge.